Anodization process for coating of magnesium surfaces

ABSTRACT

A composition that includes at least one alkali metal hydroxide, at least one alkali metal silicate, at least one alkali metal fluoride, and hydrogen peroxide. The invention also includes a method of applying an anodic coating on at least a portion of a surface of a magnesium-containing article that includes the steps of electrically connecting the article with a power source, absorbing a composition with an absorbent applicator, which is in contact with a cathode, supplying a voltage from the power source, and contacting the absorbent applicator with the article, wherein the composition comprises at least one alkali metal hydroxide, at least one alkali metal silicate, at least one alkali metal fluoride and hydrogen peroxide.

FIELD OF THE INVENTION

The invention relates to a process for anodic coating of a metal article and a composition used in forming the coating.

BACKGROUND OF THE INVENTION

Anodic coating generally refers to the electrolytic treatment of a metal, which results in the formation of stable films of oxides on the surface of the metal. The metal oxide coatings that are deposited by anodic coating methods generally are hard and have good electrical insulating properties. They can also absorb dyes and pigments, which make it possible to obtain finishes in a complete range of colors. Metals are typically anodized to protect against corrosion.

Magnesium components have long suffered from corrosion issues during the service lifetime of the parts. Corrosion occurs at mating surfaces, fastener sites and where moisture becomes trapped or pools. This is especially true near ferrous attachments and dissimilar metal inserts. Magnesium alloys have a history of use as sacrificial protectors of steel components. When the component is made of magnesium or alloys of magnesium, this natural galvanic action works against the preservation of the component Magnesium and steel together in the presence of moisture will form a galvanic cell where the magnesium becomes the anode (oxidizes) and the steel becomes the cathode (reduces). The typical protection scheme is to anodize the base metal to form a protective coating and paint the magnesium component. To minimize water infiltration the manufacturer will often assemble the fasteners and inserts after wetting them with a protective paint (wet assembly). The assembly is completed with the use of more similar washers such as those made of aluminum and insulators such as those made of plastics or by covering the steel with protective caps or by applying sealers on the exposed ferrous parts. The problem on OEM parts can be compounded when the protective coatings are machined off the mating surfaces to correct distortion and misalignment due to the wet assembly of steel liners and bearing races.

Unfortunately, the historical means of anodization requires the use of voltages and chemistries that will result in damage to ferrous metals. Anodizing has not been an option because the ferrous parts do not passivate in the process designed to anodize magnesium. The steel does not form a stable resistive coating, remaining highly conductive and acting as a preferred path for the current applied to the anodizing cell. Little current goes to the magnesium and no coating is formed. The steel is oxidized and suffers severe pitting and rusting. For these reasons, the newly bared mating surfaces often go with little or no protection and yet this location often contain the highest concentration of ferrous metal inserts. Machined OEM components often suffer premature corrosive and galvanic attack resulting in scrapping the expensive castings long before the mechanical design life is achieved. In addition corrosion issues can also arise from scratches, field repairs, service damage or tool marks that may occur during the service life of a magnesium component. Deployment of aerospace components into aggressive environments such as on aircraft carriers and other sea vessels is a major challenge to controlling corrosion. Other emerging applications of magnesium alloy components such as in the automotive, architectural, structural or military vehicle industries point to the need for more advanced anodizing methods to accommodate repairing magnesium components after the steel has been installed.

Military repair depots generally use low performance chemical conversion coatings such as DOW 7 or DOW 19 for the treatment of re-machined or repaired magnesium and scratches. Both of these systems contain chromates, which are undesirable from a safety and environmental standpoint. Chemical treatments like these can be applied to steel bearing components but their effectiveness on highly reactive magnesium alloy is minimal at best. These coatings work on other metals by galvanically sacrificing to protect the underlying component. Magnesium alloys, however, are so reactive that the mechanism of protection can be arguably reversed. Active oxidation of the magnesium component surface by the conversion coating results in the formation of a barrier oxide releasing chromates into the environment in the process. The oxide formation often chases along the surface and undermines paint in a mechanism called filiform corrosion where the stable oxide forms a resistive layer forcing the corrosive cell to seek a path of lower resistance thereby propagating the corrosion and associated pitting across the surface. The oxide coating formed by anodizing effectively eliminates filiform corrosion by providing a barrier to the propagation mechanism.

Therefore, there remains a need for methods of treating magnesium-containing parts that also contain or are attached to dissimilar metals that are more desirable from a safety, environmental and performance standpoint.

SUMMARY OF THE INVENTION

The invention includes a composition that includes at least one alkali metal hydroxide, at least one alkali metal silicate, at least one alkali metal fluoride, and hydrogen peroxide.

The invention also includes a method of applying an anodic coating on at least a portion of a surface of a magnesium-containing article that includes the steps of: electrically connecting the article with a power source, absorbing a composition with an absorbent applicator, which is in contact with a cathode, supplying a voltage from the power source, and contacting the absorbent applicator with the article, wherein the composition comprises at least one alkali metal hydroxide, at least one alkali metal silicate, at least one alkali metal fluoride and hydrogen peroxide.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts an exemplary apparatus that may be used to carry out a method of the invention.

FIG. 2 depicts another exemplary apparatus that may be used to carry out a method of the invention.

FIG. 3 depicts an anode, cathode, an article to be coated, and an absorbent applicator that can be used in one embodiment of the invention.

FIG. 4 depicts an exemplary power source that can be used in one embodiment of the invention.

FIGS. 5A, 5B, 5C, and 5D are photographs of panels coated with DOW 19 (FIG. 5A), DOW 7 (FIG. 5B), formula 12 (FIG. 5C), and formula 5 (FIG. 5D) after exposure to a neutral salt spray per the ASTM B 117 testing method for 9 hours.

FIGS. 6A, 6B, 6C, and 6D are photographs of panel coated with DOW 7 FIG. 6A), DOW 19 (FIG. 6B), formula 5 (FIG. 6C), and formula 12 (FIG. 6D), and topped with three coats of epoxy sealant, after being exposed to the scribe migration test.

FIGS. 7A and 7B are photographs of groove repairs on panels coated with formula 5C (FIG. 7A) and formula 12 (FIG. 7B).

FIGS. 8A and 8B are graphs depicting the galvanic current (FIG. 8A) and the galvanic potential (FIG. 8B) measured by electrochemical impedance spectroscopy (EIS) of a base panel and panels coated with DOW 7, DOW 19, formula 5 and formula 12.

DETAILED DESCRIPTION OF THE INVENTION Definitions

All scientific and technical terms used in this application have meanings commonly used in the art unless otherwise specified. As used in this application, the following words or phrases have the meanings specified.

As used herein, the term “about” applies to all numeric values, whether or not explicitly indicated. The term “about” generally refers to a range of numbers that one of skill in the art would consider equivalent to the recited value (i.e., having the same function or result). In many instances, the term “about” may include numbers that are rounded to the nearest significant figure.

Article To Be Coated

Methods and compositions of the invention are used to coat articles. In one embodiment of the invention, an article that can be coated is a magnesium-containing article. As used herein, the phrase “magnesium-containing article” means a metallic article having surfaces that are in whole or at least in part metallic magnesium or a magnesium alloy. In one embodiment, the article is formed of metallic magnesium or a magnesium alloy and comprises a significant amount of magnesium. In one embodiment, the article comprises a magnesium-rich alloy comprising at least about 50 wt-% magnesium. In another embodiment, the article comprises a magnesium-rich alloy comprising at least about 80 wt % magnesium.

Articles that can be coated using methods of the invention can also include articles that comprise steel. In another embodiment, articles that are coated with methods of the invention comprise iron. In one embodiment, an article to be coated comprises magnesium, or a magnesium alloy, iron, and steel. In one embodiment, the article can have paint on some portion of at least one surface thereof.

In one embodiment of the invention, the article to be coated is an article that has been machined and not yet used for its intended purpose. In another embodiment, the article to be coated is an article that has been machined, and used for its intended purpose. In yet another embodiment, the article to be coated is an article that has been machined, used for its intended purpose, and remachined.

In one embodiment of the invention, the article to be coated includes a machined magnesium surface that is in contact with an insert made of steel or another metal. In another embodiment, the article has some surface scratches or machined areas that are in need of repair.

Apparatus to Coat an Article Using a Method of the Invention

An example of an apparatus that can be used to carry out a method of the invention to coat an article 10 is depicted in FIG. 1 and includes: an anode 20, a cathode 24 equipped with an absorbent applicator 28, and a power source 30. Another embodiment of an apparatus that can be used to carry out a method of the invention to coat an article 10 is depicted in FIG. 2 and includes the components of FIG. 1 as well as a switch 40, and a microprocessor control 44. The power source 30 of FIG. 2 includes a rectifier 32 and a voltage source 36. The components of these two exemplary apparatuses are described in greater detail below.

Each apparatus includes an anode 20. The anode 20 is the positive electrode of the electrolytic cell, to which the negatively charged ions travel when an electric current is passed through the cell. The anode 20 is generally an electrically conductive material that is in contact with or can be brought in contact with the article 10 to be coated when a method of the invention is to be carried out. In one embodiment of the invention, the anode 20 can be a tool made of an electrically conductive material that is capable of holding or securing the article 10 to be coated. One of skill in the art, having read this specification, will understand that anodes of different sizes or configurations can be used, depending on the size and/or configuration of the article 10 to be coated.

In one possible embodiment, the anode 20 is an alligator clip that is capable of holding the article 10 to be coated. In another possible embodiment, the anode 20 is a clamp made of 2024 aluminum, as is commercially available. In another possible embodiment, the anode 20 is the article 10 to be coated. In such an embodiment, the article 10 to be coated is electrically connected directly to the power source 30.

Each apparatus also includes a cathode 24. The cathode 24 is the negative electrode, to which positively charged ions migrate when a current is passed through the cell. The cathode 24 is generally an electrically conductive material that can be electrically connected to the power source 30. The cathode 24 is generally configured so that the absorbent applicator 28 maintains contact with its tip. One of skill in the art, having read this specification, will understand that cathodes 24 of different sizes or configurations can be used, depending on the size and/or configuration of the article 10 to be coated, and the absorbent applicator 28.

In one embodiment, the cathode 24 is a graphite piece that is configured to fit the contours and size of the surfaces of the article to be coated. One of skill in the art, having read this specification, will understand that the sizes and configurations of the cathode 24 can be dictated, at least in part by the uniformity and features of the surfaces to be coated. For example, if the surfaces to be coated have intricate features formed therein, it may be desirable to have a cathode 24 and hence absorbent applicator 28 with very small tips that allows the absorbent applicator to be contacted more easily with the contoured surfaces of the article.

One of skill in the art will also understand, having read this specification, that the size of the cathode 24 tip will dictate, at least in part, the current that will be used with the apparatus. The current density, current per unit of contact area, is affected by the size of the cathode 24 tip. As the tip of the cathode 24 becomes smaller, the current density will increase.

Current density can vary widely within the limits of a current setting on the power supply, generally about 1 to 3 amps for a typical 1 square inch contact area As the cathode tip size is reduced, and hence the contact area, current settings may be reduced to within the range of about 0.5 to 2.0 amps to compensate this reduction in tip size. Also, the current density is generally much higher for small cathode tip areas, 1/16 square inches or less. When the cathode tip area becomes smaller, the ability to carry current at a given voltage is limited by the contact area of the cathode tip and the electrolyte conductivity. Thus, the small size of the cathode tip cannot yield a current density high enough to cause damage when voltage is held within the operational limits, i.e., generally within the range of about 0 to 50 volts peak.

The apparatus also includes an absorbent applicator 28. The absorbent applicator 28 is attached to the cathode 24. The absorbent applicator 28 can be made of any material that can absorb some of the electrolyte composition of the invention. The word absorb as used in this context means that once the absorbent applicator 28 is in contact with the electrolyte composition, some of the electrolyte composition is taken up by the absorbent applicator 28 and can be transported to the article to be coated. It is permissible for the absorbent applicator 28 to lose some of the electrolyte composition while the absorbent applicator is transported from the composition to the article, i.e., it is permissible if the absorbent applicator drips.

Examples of materials that can be used for the absorbent article 28 include, but are not limited to Dacron cloth or felt, cotton cloth, or felts. Contact can also be made by a thin stream of electrolyte or by a shielded cathode tip of metal and hard plastic shielding that is then placed in contact with either a bead of electrolyte on the surface to be coated or a pool of electrolyte in a hole in the surface. Generally any imaginable means of causing or maintaining an electrolyte bridge between the anode 20 and the cathode 24 with minimized risk of shorting the anode 20 to the cathode 24 is acceptable.

The size and shape of the absorbent applicator 28 depends at least in part on the size and configuration of the cathode 24, because the absorbent applicator 28 has to be wrapped around the cathode 24 and be able to be maintained on the cathode 24. The absorbent applicator 28 can be maintained on the cathode 24 by means of its size and/or configuration or can be maintained on the cathode 24 via a fastening mechanism.

In one embodiment of the invention, the absorbent applicator 28 and the cathode 24 are configured so that there is a tip that is smaller than the remaining portion of the absorbent applicator covered cathode. In such an embodiment, the tip portion of the absorbent applicator covered cathode can range from about 1/32 to about ¾ square inches. In another embodiment, the tip portion of the absorbent applicator covered cathode can range from about ¼ to ½ square inches.

FIG. 3 illustrates one possible embodiment of an anode 120, a cathode 124, an absorbent applicator 128, and an article to be coated 110. In this embodiment, the anode 120 includes an aluminum clamp holding the article in place. The clamp is electrically coupled to a power source (unseen). The cathode 124 and the absorbent applicator 128 include a flexible mild steel wire electrically coupled to the power source and a felt jacket. A hook was formed at the end of the wire to act as an anchor for the felt jacket One of skill will understand that the wire may be shaped in any desirable fashion.

In one possible embodiment, the flexible wire can be attached to a modified SIFCO brush wand commercially available from SIFCO Industries, Inc. The wand can be modified by attaching the flexible wire to the work end that projected out in line with the wand handle. Then, a 3 inch by 4 inch piece of Dacron felt can be cut so that, when folded in two, the wire will be completely contained in the fold and the felt will extend at least ½ inch beyond the hook end on the wire. When folded the felt will form a knife-edge ¼ inch wide and 4 inches long. The felt/wire assembly can be placed in a smooth jaw vise loosely and a high temperature hot melt glue can be applied inside the fold near the wire. The amount of hot melt glue used should be only enough to hold the felt fold fast to the wire and not so much that the felt is no longer absorbent.

One of skill in the art will also understand, having read this specification, that the configuration depicted in FIG. 3 is only one possible embodiment of a cathode 24/absorbent applicator 28 configuration. Other possible embodiments include a Q-Tip in an alligator clip, a nail with a perforated silicone tube over it or a paper clip with a steady hand.

The apparatus also includes a power source 30. Generally, any power source capable of producing a voltage to generate a current may be used. For example, in one possible embodiment, the power source 30 may include a filtered or unfiltered DC power supply. In another possible embodiment, the power source 30 may include an alternating current (AC) rheostat. In yet another possible embodiment, the power source 30 may include a solar cell. In another embodiment, the power source 30 may include a direct current (DC) power pack as shown in FIG. 4. In one possible embodiment, the power source 30 produces a peak voltage of about 50 volts. However, the invention is not limited to this voltage and other voltages such as 100 volts may also be used.

FIG. 2 illustrates another possible embodiment in which the power source 30 includes a rectifier 32 and an AC voltage source 36. The rectifier 32 functions to converts AC voltage from the AC voltage source 36 to DC voltage for output. In one embodiment, the rectifier 32 provides a pulsed DC signal that drives the deposition of the coating. The embodiment depicted in FIG. 2 also shows a switch 40 and a microprocessor control 44. In such an embodiment, the switch 40 and the rectifier 32 are placed in electrical communication with the microprocessor control 44 for the purpose of controlling the output voltage of the voltage source 36. However, the microprocessor control is not limited to controlling only the output voltage of the voltage source, and can also be used for controlling other functionality of the apparatus.

COMPOSITIONS OF THE INVENTION

Compositions of the invention include at least one alkali metal hydroxide, at least one alkali metal silicate, at least one alkali metal fluoride, and hydrogen peroxide.

As used herein, the phrase “alkali metal” means a metal in Group I of the periodic table, and includes lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). In one embodiment of the invention, alkali metal refers to Li, Na, and K. In another embodiment of the invention, alkali metal refers to Na and K. In yet another embodiment of the invention, alkali metal refers to Na.

An alkali metal hydroxide refers to a salt that includes an alkali metal cation and a hydroxide (OH⁻) anion. Examples of alkali metal hydroxides include LiOH, NaOH, KOH, RbOH, and CsOH. In one embodiment of the invention, alkali metal hydroxides include LiOH, NaOH, and KOH. In another embodiment of the invention, alkali metal hydroxides include NaOH, and KOH. In yet another embodiment of the invention, alkali metal hydroxides include NaOH.

In one embodiment of the invention, the composition includes from about 0.5 to 5 M alkali metal hydroxide. In yet another embodiment of the invention, the composition includes from about 0.75 M to about 3.75 M alkali metal hydroxide. In a further embodiment of the invention, the composition includes about 3.75 M alkali metal hydroxide.

An alkali metal silicate refers to a salt that includes an alkali metal cation, silicon (Si), oxygen (O), and can optionally contain hydrogen (H). Examples of alkali metal silicates include Li₂SiO₃; Na₂SiO₃, Na₆Si₂O₇, and Na₂Si₃O₇ with varying amounts of water; and K₂Si₂O₅ and K₂Si₃O₇ with varying amounts of water. In one embodiment of the invention, alkali metal silicates include Na₂SiO₃, Na₆Si₂O₇, and Na₂Si₃O₇ with varying amounts of water; and K₂Si₂O₅ and K₂Si₃O₇ with varying amounts of water. In another embodiment of the invention, alkali metal silicates include Na₂SiO₃, Na₆Si₂O₇, and Na₂Si₃O₇ with varying amounts of water.

In another embodiment of the invention, the alkali metal silicate is added to a composition of the invention by adding an alkali metal silicate solution. In one embodiment of the invention, the alkali metal silicate solution comprises an alkali metal hydroxide and silicon dioxide (SiO₂). In one embodiment, an alkali metal silicate solution comprises from about 5% to about 15% of an alkali metal hydroxide by weight and from about 28% to about 35% of SiO₂ by weight. In another embodiment, the alkali metal silicate solution contains from about 8% to 9% by weight of sodium, hydroxide and from about 24% to about 31% by weight of SiO₂. An example of such an alkali metal silicate solution is commercially available from PQ Corporation (Valley Forge, Pa.). In another embodiment, an alkali metal silicate solution contains about 14% sodium hydroxide by weight and about 27% SiO₂ by weight. An example of such an alkali metal silicate composition is available from Aldrich Milwaukee, Wis.).

In one embodiment of the invention, the composition includes from about 50 g/L to about 440 g/L of a sodium silicate solution that comprises from about 8 to 9% by weight of NaOH and 24 to 31% by weight of SiO₂.

An alkali metal fluoride refers to a salt that includes an alkali metal cation and a fluoride (F⁻) anion. Examples of alkali metal fluorides include LiF, NaF, KF, RbF, and CsF. In one embodiment of the invention, alkali metal fluorides include LiF, NaF, and KF. In another embodiment of the invention, alkali metal hydroxides include NaF, and KF. In yet another embodiment of the invention, alkali metal hydroxides include NaF.

In one embodiment of the invention, the composition includes from about 0.01 to 0.5 M alkali metal fluoride. In yet another embodiment of the invention, the composition includes from about 0.02 M to about 0.3 M alkali metal fluoride. In a further embodiment of the invention, the composition includes about 0.02 M alkali metal fluoride.

Compositions of the invention also include hydrogen peroxide (H₂O₂). H₂O₂ can be added to a composition of the invention by adding a solution containing an amount of H₂O₂. H₂O₂ solutions are commercially available with variable concentrations of H₂O₂, for example, 50 wt % (the weight of the H₂O₂ in the weight of the solution), 35 wt %, 30 wt %, 20 wt %, and 3 wt % (Aldrich, Milwaukee, Wis.). In one embodiment of the invention, a 20 wt % solution is utilized. The peroxide is used to passivate the steel during the electrochemical process. In one embodiment of the invention, the composition includes from about 0.2 to 3.0 M hydrogen peroxide. In yet another embodiment of the invention, the composition includes from about 0.25 M to about 2.5 M hydrogen peroxide. In a further embodiment of the invention, the composition includes about 0.2 M to about 0.3 M hydrogen peroxide, and another embodiment of the invention 0.29 M hydrogen peroxide is used.

One embodiment of the invention includes a composition having sodium hydroxide, sodium silicate, sodium fluoride, and hydrogen peroxide. It is thought, but not relied upon that a composition that includes all sodium salts limits the heat of solution when the composition is used in methods of the invention. This may be due, at least in part, to the greater heat capacity of aqueous sodium ions (C.°_(p)Na⁺ _((aq))=11.1 cal/° mol) than aqueous potassium ions (C.°_(p)K⁺ _((aq))=5.2 cal/° mol). It is also thought, but not relied upon, that the combination of the sodium anions and the high pH may function at least in part to decrease the maximum deposition voltage, which could be considered an advantage of a method of the invention.

One embodiment of the invention is a composition that includes 3.75 M sodium hydroxide, 50 g/L of a sodium silicate solution (about 8 to about 9 wt % NaOH and about 24 to about 31 wt % SiO₂), 0.02 M sodium fluoride, and 0.29 M hydrogen peroxide. Another embodiment of the invention is a composition that includes 0.75 M sodium hydroxide, 50 g/L of a sodium silicate solution (about 8 to about 9 wt % NaOH and about 24 to about 31 wt % SiO₂), 0.02 M sodium fluoride, and 0.29 M hydrogen peroxide. Yet another embodiment of the invention is a composition that includes 0.75 M sodium hydroxide, 440 g/L of a sodium silicate solution (about 8 to about 9 wt % NaOH and about 24 to about 31 wt % SiO₂), 0.02 M sodium fluoride, and 2.35 M hydrogen peroxide.

In one embodiment, the pH of an electrolyte composition of the invention is from about 10 to about 13.5.

METHODS OF THE INVENTION

The invention includes methods for anodic coating of an article. One advantage of the invention is that, unlike the prior art, these methods do not require maskings of steel or adjacent finishes.

The invention includes methods of applying a coating on an article comprising the steps of: electrically connecting the uncoated article with a power source, absorbing an electrolyte composition with an absorbent applicator that is in contact with a cathode, supplying a voltage from the power source, and contacting the absorbent applicator with the article in order to apply the coating. The step of supplying the voltage may be performed either before or after absorbing the electrolyte composition with the applicator.

In one embodiment of the invention, the step of electrically connecting the uncoated article with a power source is accomplished through the use of an anode. For example, as shown in FIG. 3, the anode can be a clamp that is configured to hold the article to be coated. The anode is then electrically connected to the power source. In another embodiment of the invention the article itself can be electrically connected to the power source.

In one embodiment of the invention, the step of absorbing an electrolyte composition with an absorbent applicator that is in contact with a cathode is accomplished by, for example, dipping the absorbent applicator into a container holding the electrolyte composition. In another embodiment, the electrolyte composition can be sprayed onto the absorbent applicator or onto the article in the vicinity of the absorbent applicator.

In one embodiment of the invention, the step of supplying a voltage from the power source is accomplished by turning the power source on. The power source may be set to an open circuit, and a peak voltage may be set to about 100 V or less. Although it is not necessary to control the voltage of the cell, the voltage differential can be monitored and a peak voltage can be set by configuring the power source in another embodiment of the invention. In a preferred embodiment, the peak voltage is set to about 50 V or less. In another embodiment, the voltage can vary from between about 10 V to about 100 V. Embodiments of the invention where the peak voltage is set to about 100 V or less, and especially to about 50 V or less, offer advantages because they are generally safer for operators due to the lower voltages.

Another parameter that is set on the power source is the current. The current is estimated based at least in part on the size of the cathode and the absorbent applicator and the rate at which the coating is to be applied. In one embodiment of the invention, the current ranges from about 0.4 A to about 3 A.

In one embodiment of the invention, the step of contacting the absorbent applicator with the article is accomplished by placing the absorbent applicator on a portion of the article to be coated. In another embodiment of the invention, the absorbent applicator is in constant motion on the surface of the article to be coated. The rate at which the absorbent applicator is moved across the surface of the article to be coated can be varied based, at least in part, on the rate at which the coating is to be applied, the extent of the surface area to be coated, and the uniformity of the coating formed.

In an embodiment where the article to be coated includes steel, the absorbent applicator can be used to “scribble” around the steel. As used herein, the word “scribble” means that the absorbent applicator is used in such a way that only the tip of the absorbent applicator touches the article. It is thought, but not relied upon, that scribbling around steel portions of the article can facilitate coating formation on the adjacent magnesium by increasing the current density while the coating is being applied.

In another embodiment of the invention, the step of absorbing the electrolyte composition in the absorbent applicator can be repeated. This may be necessary or desired when the coating process has used or consumed some or all of the electrolyte composition that was originally absorbed in the absorbent applicator. In one embodiment of the invention, this step can be accomplished by redipping the absorbent applicator into the container with the electrolyte composition. The absorbent applicator may also dry out due to heat that can be generated during the coating process. Generally, when the absorbent applicator dries out, the coating will not form. Therefore, it may be necessary to absorb more of the electrolyte composition into the absorbent applicator.

An exemplary embodiment of a method of the invention includes the following steps. First, the anode is connected to the positive lead of the power source. Next the absorbent applicator on the cathode is connected to the negative lead of the power source. After both the anode and the cathode are connected to the power source, it is turned on. The peak voltage of the power source is set to about 50 V or less, and the current is adjusted to between about 0.4 to about 3 Amps depending on the area of the absorbent applicator of the cathode and the rate at which the coating is to be applied. The absorbent applicator is brought into contact with the article to be coated and the applicator is moved across the article. The absorbent applicator is dipped into the electrolyte solution when the absorbent applicator becomes dry.

WORKING EXAMPLES

The following examples provide nonlimiting illustrations of various embodiments of the invention.

Example 1 Initial Formulation of Electrolyte Solution

A composition was prepared having the following components:

2000 g H₂O

294 g H₂O₂ solution (35% H₂O₂ solution) 67 g potassium silicate solution (20% potassium silicate solution by weight) 9 g NaOH (dry bead, technical grade) 109 g NaF (powder, technical grade). The pH of the solution was about 10.5. The composition was utilized to anodically coat magnesium, aluminum, titanium, and other valve metal pieces that contained steel, using the following process.

The power supply, a Xantrex model XHR 100-6 DC voltage source with a filtered DC output of up to 100 volts and 6 amps (Xantrex Technology, Inc., Arlington, Va.), was connected to the cathode, which was a modified SIFCO brush wand (SIFCO Industries, Inc., Cleveland, Ohio) using a steel wire wrapped in felt. The anode was a clamp made from 2024 aluminum. FIG. 3 shows the cathode (the SIFCO wand wrapped in felt) and the anode.

Each metal piece was coated by dipping the felt tip of the brush head into the electrolyte solution (composition given above) and lightly placing it on the surface of the piece. With the voltage applied, the brush head was moved in a circular motion on the piece. Throughout the coating process, the felt was kept saturated by repeatedly dipping the brush head into the composition and then reapplying it to the metal piece. The working voltage ranged from 15 to 18 volts on average and reached a peak voltage of 48 V. The current density ranged from 30 to 300 amps per square foot (ASF) of contact area The contact area of the pieces ranged from 0.25 to 0.5 in². The amperage set limit was between about 0.1 and 1.6 amps.

The process was determined to be complete when each piece was covered with what visually appeared to be a uniform coating. The coating process did not appear to damage the paint interface, but did not work well around the steel portions of the metal pieces.

A further experiment was done in which the pH of the solution was adjusted via the addition of more NaOH to as high as pH 13.5. The pH 13.5 solution worked better around the steel portions of the pieces, formed good coatings, and did not appear to damage the paint interface.

Example 2 Comparative Example Composition Including All Sodium Components and No Hydrogen Peroxide

A composition was prepared with the following components:

100 g water 20 g sodium silicate (20% sodium silicate solution by weight) 8 g sodium hydroxide (dry, technical grade) 29 g sodium fluoride (powder, technical grade) The pH of the solution was about 13.4. The electrolyte solution was used to anodically coat magnesium, aluminum, titanium, and other valve metals that contained steel according to the process described in Example 1.

The working voltage ranged from 15 to 20 volts, on average, and reached a peak voltage of 48 volts. The amperage set limit was between about 0.5 and 0.85 amps on a 0.5 in² wand tip. The electrolyte saturated felt was allowed to get hot and a vapor was produced.

The metal pieces were coated without damaging the steel. The process left a thin coating near the steel, but the coverage was poor. The process did not damage the paint interface. There was no visible coating on the stainless steel portions.

From this experiment, it was determined that using a high pH sodium based solution will not damage the steel and results in low voltages. However, the solution did not produce thick coatings up to the steel.

Example 3 Electrolyte Composition With All Sodium Components and Hydrogen Peroxide

A composition was prepared having the following components:

900 g water 20 g hydrogen peroxide solution (35% H₂O₂ solution) 230 g sodium silicate solution (20% sodium silicate solution by weight) 25 g sodium hydroxide (dry, technical grade) 3 g sodium fluoride (powder, technical grade) The pH of the solution was about 13.7. The electrolyte solution composition was utilized to anodically coat magnesium, aluminum, titanium, and other valve metal pieces that contained steel using a process according to Example 1.

The working voltage ranged from 15 to 20 volts on average, and reached a peak voltage of about 48 volts. The amperage set limit was at from 0.5 to 0.85 amps on a 0.5 in² wand tip. The electrolyte saturated felt was allowed to get hot and a vapor was produced.

The method produced a very good coating around the steel, and did not damage the paint interface. The process left an opaque coating on mild steel and on stainless steel. The coating in this example was built rapidly and the voltage did not build as the thickness did, as would be expected.

It was further found in this example that cooling the electrolyte solution to about room temperature helps control the vapor production, but slows the rate of coating deposition and produces less uniform coatings. It should also be noted that cooling a composition such as this to below about 50° C. could cause irreversible precipitation.

From this experiment, it was determined that a high pH, high silicate concentration composition containing hydrogen peroxide produced the desired result of quickly built, uniform coatings in a mixed metal environment.

Example 4 Optimization of Silicate Based Electrolyte

The initial goal of the investigation was to create an anodic coating on the magnesium near and up to steel inserts without damaging the steel. The purpose of this example was to optimize the electrolyte solution determined in the above examples to yield useful properties such as hardness and paint adhesion.

The compositions tested are shown in Table 1 below:

TABLE 1 Sodium Silicate Formula # NaOH (g/L) Solution*(g/L) NaF (g/L) H₂O₂ (g/L) 1 30 440 1 16 2 30 50 12 16 3 90 245 6.5 9 4 150 440 1 2 5 30 50 1 2 6 90 245 6.5 9 7 150 50 12 2 8 150 50 1 16 9 150 440 12 16 10 30 440 12 2 11 30 440 12 16 12 150 50 1 2 *The sodium silicate solution had from 8 to 9 wt % NaOH and 24 to 31 wt. % SiO₂ (PQ Corporation, Valley Forge, PA).

Samples

The magnesium alloy, ZE41 (AMS 4439E) was used for the investigation due to its popularity as an aerospace alloy and its rapid corrosion rate. Blocks of ZE41 metal were purchased from Fansteel Wellman Dynamics (Creston, Iowa), then cut down into 4″×5″×⅜″ test panels. Pieces of the magnesium alloys, WE43 (AMS 4427A) and QE22 (AMS 4418F), were also purchased from Fansteel Wellman Dynamics and remachined in the same manner. The chemicals used were reagent or electronic grade and working solutions were diluted using in-house deionized water, with a maximum conductivity of about 10 μS.

Coating Process

The equipment used was the same as in Example 1.

The electrolyte was chilled to about 8-10° C. prior to deposition on the test panel. The rectifier was set at 100 V DC max and 0.6 amps max throughout the deposition process. For each test panel, the felt tip of the brush head was dipped into the chilled electrolyte and lightly placed on the surface of the panel. With the voltage applied, the brush head was moved in a circular motion on the panel. During the coating process, the brush head was placed in the chilled electrolyte on a regular basis in order to saturate the felt. The brush head was reapplied to the test panel and the coating process was continued. Deposition of the coating was halted when the current output fell to 0.08 amps at the maximum voltage. Each electrolyte solution had a characteristic maximum deposition voltage (shown in Table 2 below).

Each electrolyte solution (formula numbers 1-12) was applied to three identical panels. After application of the coating, two of the three panels were painted with a two-part epoxy (Rockhard Clear Stoving Enamel, Indestructible Paint, Ltd., Birmingham, UK) while the third panel was left unpainted.

The panels were post-treated by immersing the panels into a beaker containing 0.5 M tri-sodium phosphate (TSP) for about 12 to 15 seconds. This post-treatment was followed by 2 to 3 minutes of rinsing the panels with deionized water and drying them with oil free compressed air.

Comparative Samples

For comparison, DOW 19 panels were coated using the standard method given in the DOW Chemical Handbook (The DOW Chemical Company. “Magnesium: Operations in Magnesium Finishing”, No. 141-479-86R. Midland, Mich., 1990). The DOW 7, HAE and DOW 17 coatings were applied using standard methods (The DOW Chemical Company. “Magnesium: Operations in Magnesium Finishing”, No. 141-479-86R. Midland, Mich., 1990). For paint adhesion studies, the panels were painted with Rockhard Clear Stoving Enamel. The paint was applied and cured via the standard method of application given on the paint.

Salt Spray and Salt Fog Testing Done on Samples and Comparative Samples

The unpainted anodized panels, the bare ZE41 panels, the ZE41 panels coated with the electrolyte compositions, and the ZE41 panels coated with DOW7 and DOW19 were exposed to neutral salt spray per the ASTM B117 testing method for nine (9) hours, than rated per ASTM D1654 Procedure B. ASTM D1654 B uses a 0 to 10 rating scale where 10 means that no corrosion occurred. A rating of 9 means that 0.5% of the surface was corroded. The results for this test are listed as the stand-alone salt spray test in Table 2. Salt spray panels for DOW 7, DOW 19, Formulas #5 and #12 are seen in FIGS. 5A through 5D. More specifically, FIG. 5A shows a panel coated with DOW 19, FIG. 5B shows a panel coated with DOW 7, FIG. 5C shows a panel coated with formula 12, and FIG. 5D shows a panel coated with formula 5.

Further examination of the formula combinations used in Formulas #1, 2 and 5 suggested that several combinations that were not in the original design might generate good salt spray responses. The new combinations, designated Formulas #11 and 12, were deposited on ZE41 panels and exposed for 9 hours in salt fog. These additional formulas received ratings of nine (9). Formula #12 was included in the paint adhesion evaluation.

TABLE 2 Stand Alone Salt Spray Rating Wet Scribe Peak Average (9 hrs) per Dry Immersion Migration Taber voltage Voltage Thickness ASTM D 1654 Tape Tape Salt Fog Adhesion Formula #* (DC) (DC) (mils) Procedure B Adhesion Adhesion 168 hrs (cycles/mil) 1 200 85 0.22/0.30 9/9 5/5 5/5 5/9 1154 2 278 80 0.07/0.09 9/9 5/5 5/5 9/9 8000 3 113 26 0.13 8 5 5 7 220 4 104 22 0.09 6 5 2 5 233 5 252 60 0.06/0.06 9/9 5/5 5/5 7/9 2250 6 139 32 0.15 7 5 3 7 536 7 152 18 0.06 7 5 5 4 1167 8 87 13 0.03 8 5 4 6 900 9 152 21 0.10 6 5 0 6 233 10 226 70 0.30 8 5 1 9 236 11 245 82 0.22/0.20 9/8 5/5 5/5 9/9 491 12 85 16 0.02/0.02 9/9 5/5 5/5 9/9 2750 Bare ZE41 0 0 DOW 7  0.20 7/0 5 5 5 1316 DOW 19 0.10 0 5 5 5 80 *Sec Table 1 for Compositions of Formulas

Paint Adhesion Testing

Formulas # 5 and 12 were chosen for paint adhesion evaluation. Paint adhesion was evaluated by applying three (3) coats of a two-part epoxy sealant to ZE41 panels coated with Silicate Formulas #5 and 12 as well as DOW 7 and DOW 19. The paint adhesion tests were: dry tape test, warm water immersion (96 hours)/scribed tape test and scribed migration in salt spray (168 hours). The results are summarized in Table 3 and scribe migration pictures are shown in FIGS. 6A through 6D. More specifically, FIG. 6A shows a panel coated in DOW 7, FIG. 6B shows a panel coated in DOW 19, FIG. 6C shows a panel coated in formula 5, and FIG. 6D shows a panel coated in formula 12.

Two of the three painted panels were tested for paint adhesion dry tape pull as specified in ASTM D3359 Method B. In addition, one of the painted specimens was immersed in warm water (38° C.) for 96 hours (ASTM D870). After a five (5) minute cool down, a razor blade was used to score a grid on the panel, reaching bare metal with each scribe line. A strip of masking tape was pressed firmly on the grid and then the tape was pulled off quickly while maintaining an angle of 90°. Paint adhesion rating was based on percentage of the coating remaining on the surface as defined in ASTM D3359 Method B. A rating of 5 means no paint was removed. A rating of 0 means 100% of the paint was removed.

A painted panel was used to determine scribe migration. The panel was prepared by using a carbide scribe to place large “X” on the panel. The scribe was inspected to verify the exposure of bare metal. The panels were placed in a neutral salt spray for one week (168 hours), than rated per ASTM D1654 Procedure A. A rating of 9 means the median width of the scribe line did not increase more than 0.5 mm when the area of corrosion found in the scribe is averaged over the length of the scribe.

TABLE 3 Dry Tape Wet Immersion Scribe Migration Adhesion Rating Tape Adhesion Rating after 168 hrs per ASTM D3359 Rating per ASTM Salt Spray per ASTM Formula # Method B D3359 Method B D1654 Procedure A  5 5 5 9 12 5 5 9 DOW 7  5 5 9 DOW 19 5 5 9

Example 5 Compatibility with Ferrous Materials

The coating performance of the five silicate based electrolytes were all equivalent based on the test criteria above. Following the performance evaluation, the issue of the ability to apply an anodized coating on or near ferrous materials was revisited. The ferrous compatibility issue was investigated by applying the electrolytes to a scrap helicopter housing made from ZE41. Mating surfaces were cleaned and stripped to bare metal prior to application of silicate coatings. The five different silicate electrolytes were applied to the mating surfaces and the interactions around the ferrous inserts and bearing races were carefully observed.

Typical anodized systems will not exhibit an ability to build voltage during coating when ferrous material is present. Since the ferrous material does not build a passive stable film in this type of anodic process, no resistance builds and the process is essentially “shorted-out”. Anodizing the ferrous material in an electrolyte typical in the art will lead to rapid oxidation and pitting.

Three of the five electrolytes (Formulas #1, 2 and 11) had a detrimental effect on ferrous material, which was defined as a rapid decrease in voltage followed by rapid oxidation (rust formation) of ferrous material. Formulas #5 and 12 proved to be compatible with ferrous material, which was characterized by a 50% to 75% fall in voltage for formula #5 and a 5% to 25% fall in voltage for formula #12. Formula #5 would allow brief (15-20 second) contact with ferrous material but prolonged contact (1-2 minutes) would result in corrosion formation. Formula #12 would allow prolonged contact (2-3 minutes) without any visible signs of corrosion.

Example 6 Compatibility with Standard Anodized Coatings

The vast majority of new magnesium aerospace components are anodized prior to painting, installation of ferrous inserts and final machining. The ability to selectively coat bare magnesium next to common anodic coatings was evaluated using test panels. For this experiment, ZE41 test panels were anodized with several common anodization treatments: HAE, DOW 17 and Tagnite 8200. All panels were painted with a two-part epoxy sealant. Then ¼″ and ½″ grooves were machined into the panels exposing bare magnesium. The silicate electrolytes, Formulas #5 and 12, were used to selectively anodize the bare magnesium grooves. During and after the coating deposition, the interface between the original coating/paint and the repair was carefully examined for evidence of degradation of the paint or original anodize coating by visual and optical microscopy. There was no evidence of coating/paint degradation from the application of Formula #5 or 12 silicate electrolytes as seen in FIGS. 7A and 7B. More specifically, FIG. 7A shows the results from Formula #5 and FIG. 7B shows the results from Formula #12.

Example 7 Protection of Additional Magnesium Alloys

The ability of Silicate Formulas #5 and 12 to protect other magnesium alloys was verified by applying these electrolytes to the magnesium alloys, WE43 and QE22. All the testing listed above was repeated on these alloys and the results were similar to those observed with the ZE41 alloy.

Example 8 Galvanic Corrosion Studies

ZE41 test panels were coated with Silicate Formulas #5 and 12 along with DOW 7 and DOW 19 and the electrochemical interaction between the coated test panels and a coupled cadmium plated steel bolt/washer/nut system was investigated. In this study, the four coatings were subjected to Electrochemical Impedance Spectroscopy (EIS) and the corrosion potential (CP) and galvanic current (GC) measurements were reported. EIS testing was done using a sodium sulfate solution as the electrolyte.

FIGS. 8A and 8B show the results. FIG. 8A depicts the GC readings obtained from testing the four panels and FIG. 8B depicts the CP readings. All readings are negative values. Thus, the higher data points on the chart are closer to zero, at which there is no potential or current flow. The results consistently showed formula #12 with the least negative potential and current readings. Bare metal ranked second and Formula #5 was third. Both Dow formulas ranked significantly below formula #5. It could be concluded based on this data that Formulas #5 and #12 can protect the underlying magnesium from corrosion under the Na₂SO₄ electrolyte, as well as, and perhaps, even better than DOW 7 and DOW 19.

The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended. 

1.-15. (canceled)
 16. A method of applying an anodic coating on at least a portion of a surface of a magnesium-containing article comprising: electrically connecting the article with a power source; absorbing a composition with an absorbent applicator, which is in contact with a cathode; supplying a voltage from the power source; and contacting the absorbent applicator with the article, wherein the composition comprises at least one alkali metal hydroxide, at least one alkali metal silicate, at least one alkali metal fluoride, and hydrogen peroxide.
 17. The method of claim 16, wherein the uncoated article is electrically connected to the power source through an anode.
 18. The method of claim 16, wherein said step of absorbing the composition with the absorbent applicator is accomplished by dipping the absorbent applicator into a container holding the composition.
 19. The method of claim 16, wherein the supplied voltage is less than or equal to about 100 V.
 20. The method of claim 16, wherein the supplied voltage is less than or equal to about 50 V.
 21. The method of claim 16, further comprising moving the absorbent applicator across the surface of the article to be coated.
 22. The method of claim 16, further comprising repeating the steps of absorbing a composition with an absorbent applicator, which is in contact with a cathode and contacting the absorbent applicator with the article.
 23. The method of claim 16, wherein the composition comprises sodium hydroxide, sodium silicate, sodium fluoride, and hydrogen peroxide.
 24. The method of claim 16, wherein said article further comprises steel. 25.-28. (canceled)
 29. A method of applying an anodic coating on at least a portion of a surface of an article, the article including a magnesium-containing portion and a dissimilar metal or metals attached to the magnesium-containing portion, the method comprising: electrically connecting the article with a power source; absorbing a composition with an absorbent applicator, which is in contact with a cathode; supplying a voltage from the power source; and contacting the absorbent applicator with the article, wherein the composition comprises: at least one alkali metal hydroxide having a concentration of about 0.5 M to about 5 M; at least one alkali metal silicate having a concentration of about 50 g/L to about 440 g/L; at least one alkali metal fluoride having a concentration of about 0.01 M to about 0.5 M; and hydrogen peroxide having a concentration of about 0.2 M to about 3.0 M.
 30. The method of claim 29, wherein the supplied voltage is less than or equal to about 100 V.
 31. The method of claim 29, wherein the supplied voltage is less than or equal to about 50 V.
 32. The method of claim 29, wherein the dissimilar metal or metals is a ferrous metal.
 33. The method of claim 29, wherein the dissimilar metal is steel.
 34. The method of claim 16, wherein the article further comprises a ferrous metal. 